Playing computer games during the breaks would not certainly be productive to do between the long study hours. Watching TV will also not be a good idea (it will jam the brain further, I guess). Similarly reading novels may pose distraction.

Would be different for different people, but in your case, it seems you have a hard time focusing back on work again if you do something stimulating in the break, so I guess the answer here is - something relaxing, but not distracting.

Listening to music, taking a nap, or a walk, or you could try meditating.

You do need to relax. I don't think games and TV are unproductive; some of the smartest people I know would watch TV or play games between study hours! A consistent reward makes it easier to focus on things.

If you want something better, yeah, talk to people. Or do housework - sweep/vacuum your house, do laundry, shop for grocery ingredients. A lot of students say they don't have time to cook.. well, this is a good time to cook.